Storing energy in the form of compressed gas has a long history and components tend to be well tested, reliable, and have long lifetimes. The general principle of compressed-gas energy storage (CAES) is that generated energy (e.g. electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas remains at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, one in which no heat is exchanged between the gas and its environment, because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during compression and expansion, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. patent application Ser. Nos. 12/421,057 (the '057 application) and 12/639,703 (the '703 application), the disclosures of which are hereby incorporated herein by reference in their entireties. The '057 and '703 applications disclose systems and methods for expanding gas isothermally in staged hydraulic/pneumatic cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas is used to drive a hydraulic pump/motor subsystem that produces electricity.
Additionally, in various systems disclosed in the '057 and '703 applications, reciprocal motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of means, for example as disclosed in U.S. Provisional Patent Application Nos. 61/257,583 (the '583 application), 61/287,938 (the '938 application), and 61/310,070 (the '070 application), the disclosures of which are hereby incorporated herein by reference in their entireties.
The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
Various embodiments described in the '057 application involve several energy conversion stages: during compression, electrical energy is converted to rotary motion in an electric motor, then converted to hydraulic fluid flow in a hydraulic pump, then converted to linear motion of a piston in a hydraulic-pneumatic cylinder assembly, then converted to mechanical potential energy in the form of compressed gas.
Conversely, during retrieval of energy from storage by gas expansion, the potential energy of pressurized gas is converted to linear motion of a piston in a hydraulic-pneumatic cylinder assembly, then converted to hydraulic fluid flow through a hydraulic motor to produce rotary mechanical motion, then converted to electricity using a rotary electric generator.
Both these processes—storage and retrieval of energy—present opportunities for improvement of efficiency, reliability, and cost-effectiveness. One such opportunity is created by the fact that the pressure in any pressurized gas-storage reservoir tends to decrease as gas is released from it. Moreover, when discrete quantities or installments of gas are released into the pneumatic side of a pneumatic-hydraulic intensifier for the purpose of driving its piston, as described in the '057 application, the force acting on the piston declines as the installment of gas expands. The result, in a system where the hydraulic fluid pressurized by the intensifier is use to drive a hydraulic motor/pump, is variable hydraulic pressure driving the motor/pump. For a fixed-displacement hydraulic motor/pump whose shaft is affixed to that of an electric motor/generator, this will result in variable electrical power output from the system. This is disadvantageous because (a) it is desirable that the power output of an energy storage system be approximately constant (b) a hydraulic motor/pump or electric motor/generator runs most efficiently over a limited power range. Widely varying hydraulic pressure is therefore intrinsically undesirable. A variable-displacement hydraulic motor may be used to achieve constant power output despite varying hydraulic pressure over a certain range of pressures, yet the pressure range must still be limited to maximize efficiency.
Another opportunity is presented by the fact that pneumatic-hydraulic intensifier cylinders that may be utilized in systems described in the '057 and '703 applications may be custom-designed and built, and may therefore be difficult to service and maintain. Energy-storage systems utilizing more standard components that enable more efficient maintenance through, e.g., straightforward access to seals, would increase up-time and decrease total cost-of-ownership.